1. Field of the Invention
The present invention relates to the field of filter cleaning systems. In particular, the present invention relates to a self-cleaning air filter and cooling system, preferably, for electronic equipment enclosures.
2. Background Information
Telecommunications equipment can be installed in central office environments that are relatively clean and subject to limited temperature extremes. In response to these conditions, such telecommunications equipment was generally designed with relatively open, vertical printed circuit board orientations. These circuit boards were fitted into a card cage and utilized a baffle arrangement configured to promote convective flow of ambient air through the equipment to expel internally-generated heat that could otherwise damage the equipment. This approach was appropriate for the intended environment, required no maintenance, and provided good equipment life expectancies.
Increasing emphasis has been placed on the control of flame-spread during mandated laboratory test fires originating inside and outside of the equipment volumes. A trend has also developed in which equipment complexity has increased while equipment volumes have been reduced. These competing objectives have pushed the convective air flow equipment cooling approach to the engineering limit. Additionally, a significant portion of new equipment is now installed at remote terminals that have relatively dirty environments and that are subject to relatively more severe temperature extremes, for example, −40° C. to +65° C. Under these more harsh conditions, the simple convective approach to expelling equipment heat is inadequate.
One method of addressing the problem of heat dissipation is to enclose the telecommunications equipment in a box and add a small fan or blower inside the equipment volume to circulate the captive air. Such a configuration ensures a reasonably uniform air temperature throughout the interior of the equipment volume. In such a configuration, heat is expelled by conduction through the metallic walls of the equipment housing, and subsequently dissipated to the environment by convective cooling of the entire equipment exterior surface. Since no outside air circulates into the equipment interior, concerns regarding equipment flammability or susceptibility to contamination are reduced. At the same time, the forced circulation in the interior of the equipment volume allows denser circuitry, thereby reducing equipment volume. However, the available equipment surface area is limited, so that even this approach to heat dissipation can be inadequate.
A method of mitigating the exterior surface area limitation includes an additional mechanism to separately circulate ambient air over the exterior surface of the equipment, so that air inside and outside the equipment enclosure do not intermix. In this way, the thermal resistance from the outside surface of the enclosure to the ambient air is greatly reduced, compared with unaided convective cooling. This approach results in the construction of a somewhat complex equipment enclosure with a mechanism to separately move the air interior to and exterior to the equipment enclosure. Such a mechanism can exacerbate the total equipment energy usage.
To avoid contamination, the approach of simply passing external ambient air directly through the equipment requires the use of a filter. The use of a filter, in turn, requires that some maintenance interval be specified consistent with dirt level and filter size constraints. Accordingly, although current equipment standards permit a fan in conjunction with a filter and a specified maintenance interval, in practice, the filter will rarely be changed without regard to the maintenance interval specified, unless there is an equipment failure.
To solve these and other related problems, self-cleaning filter systems have been developed. For example, U.S. Pat. No. 4,971,026 (the '026 patent) discloses a continuous self-cleaning filter for use in a warm air furnace. The system disclosed in the '026 patent requires multiple air flow paths configured with multiple blowers and ducting arrangements, as well as the use of a rotor/stator with radially-rotatable blades. However, such systems are relatively complex and expensive.
Thus, there is a need for a continuous self-cleaning filter for use with various equipment, including telecommunications equipment, that requires little or no maintenance and that is less complex and less expensive than known self-cleaning filter systems.